Polish-Soviet War

The Polish-Soviet War (14 February 1919-18 March 1921) was a war between Poland and Soviet Russia over the control of western Ukraine and Belarus. The war resulted in Poland retaking western Ukraine and western Belarus, while the Soviets took over eastern Ukraine and eastern Belarus. The war was a decisive moment of the Interwar period, as the Soviets gave up their goal of international revolution; Poland's victory prevented the Soviets from assisting the revolutions in Germany, Hungary, and Romania, and took large swathes of Soviet territory.

Background
After World War I, Polish general Jozef Pilsudski felt the time was right to expand Poland's borders as far east as possible, to be followed by the creation of an "Intermarium" federation to serve as a bulwark against the re-emergence of German and Russian nationalism. Meanwhile, Bolshevik leader Vladimir Lenin saw Poland as the bridge the Red Army had to cross to assist other communist movements and bring about other European revolutions. In 1918-1919, the Soviets launched a westward offensive into areas that were being abandoned by the Imperial German Army following the end of the Great War. On 10 December 1918, Soviet forces entered Minsk and put down the Belarusian People's Republic, proclaiming the Byelorussian SSR in Minsk on 8 January 1919. At the same time, Polish and Belarusian self-defense units sprung up across western Belarus, and the Poles and Soviets struggled over Vilna in the first week of 1919. After the Polish militia was forced from the city, the Polish Army began to send forces to assist the self-defense forces, making open conflict inevitable. On 12 February 1919, Soviet general Jukums Vacietis ordered a "reconnaissance in depth" as far as Tilsit, Brest-Litovsk, Kowel, and Rivne, and also ordered the securing of the railroad junctions at Vilna, Lida, Baranowicze, and Luninets.

War
On 14 February, the first clashes occurred around Bereza Kartuska and Mosty in what is now Belarus. By late February, the Soviet offensive had come to a halt, as the Soviets underestimated the strength of the Polish forces. The Red Army's half-hearted attacks were repelled, and the Polish counterattack and the Soviet offensive started at the same time, leading to an escalation in troop numbers. In April 1919, the Soviets captured Grodno and Vilna, but they lost the two cities to a Polish counterattack later that month. On 8 August 1919, the Poles took Minsk, and they then took Babruysk on the Berezina River and reached the Daugava River on 2 October. The Soviets had been weakened by the advances of Anton Denikin's White Army in the Baltics, enabling the Poles to expand eastward with ease.

In 1920, Ukrainian nationalist leader Szymon Petlyura allied with the Poles, Ukraine's former enemies, contributing 15,000 Ukrainian troops to the Polish army. In January, the Poles concentrated 700,000 troops near the Berezina River and in Belarus, preparing to launch an offensive against Kiev in the Ukrainian SSR. At the same time, the Soviet Southwestern Front had a strength of only 82,847 soldiers. On 5 January 1920, the Poles conquered Daugavpils before turning it over to the Latvians. On 24 April, the Poles began their Kiev Offensive, and Pilsudski told the Ukrainians that Polish troops would only occupy Ukraine until a legal government took over its own territory, although many Ukrainians were both anti-Polish and anti-Bolshevik. On 7 May 1920, after token resistence, the Poles and Ukrainians entered an abandoned Kiev. However, their advance was met by Red Army counterattacks on 29 May. In the north, the Soviets recaptured territories between the Western Dvina and Berezina rivers, and the Polish-Ukrainian front in the south was broken by attacks from Semyon Budyonny's 1st Cavalry Army on 24 May. By 10 June, the Poles were retreating across the entire front, and Kiev fell to the Soviets on 13 June.

The Soviet counterattack was aided by the former Czarist commander-in-chief Aleksei Brusilov's appeal for all former Imperial Russian Army soldiers to join the Bolsheviks in defending Russia; 14,000 officers and over 100,000 deserters enlisted in or returned to the Red Army. On 4 July, the Soviet general Mikhail Tukhachevsky's Northwest Front (consisting of 108,000 infantry and 11,000 cavalry) launched an offensive against the 120,000 Polish troops left to protect the 200-mile front. By 7 July 1920, the Poles were in retreat, although thteir strong defense of their lines prevented the Soviets from pushing the defenders into the Pinsk Marshes. On 14 July, the Lithuanians recaptured Vilna (Vilnius) from the Poles, driving them further west. In the south, Budyonny's Soviet forces captured Brody and approached Lwow and Zamosc. Grodno fell on 19 July, followed by Brest-Litovsk on 1 August. By 2 August, the Red Army was only 60 miles from Warsaw, and the Poles were pushed out of Ukraine. From July to September, the Poles were besieged in Lwow, and five Soviet armies approached Warsaw.

On 12 August 1920, the Soviets began their assault on Warsaw. The Poles succeeded in decrypting Red Army radio messages, however, and they left the northern front along the Vistula River under-defended, leading to Tukhachevsky leaving only token forces to guard the area south of Warsaw. The Soviets were also disadvantaged by the effective neutralization of Budyonny's army at Lwow, while the insubordinate Joseph Stalin decided to keep his Lviv Front engaged at Lwow rather than advance to assist Tukhachevsky at Warsaw. The Polish counterattack on 14 August, led by Wladyslaw Sikorski, halted the Soviet advance toward Warsaw and Modlin and turned it into a retreat, and Marshal Pilsudski's Reserve Army joined the Polish counter-offensive. Soviet armies in the center of the front fell into chaos, and the Bolshevik retreat was thrown into disarray by communication failures. At Komarow on 30 August-2 September 1920, the last major cavalry battle in military history was fought, with Budyonny's 1st Cavalry Army being destroyed. Tukhachevsky organized a new defensive line centered at Grodno, but this line was broken at the Battle of the Niemen River on 15-25 September, and the Poles reached the Ternopil-Dubno-Minsk-Drissa line. In the south, the Ukrainians took control of the left bank of the Zbruch river.

On 5 October, the Poles and Soviets agreed on peace terms, but long negotiations would continue. In November, Petlyura's 23,000 Ukrainian soldiers launched an offensive into Ukraine, but they were attacked by the Bolsheviks on 10 November and driven back into Polish-controlled territory on 21 November. The Polish-Soviet ceasefire lasted until the Peace of Riga was signed on 18 March 1921, dividing the disputed territories in Ukraine and Belarus between Poland and Russia. The Poles gained 124 miles of new territory east of its Curzon Line border, but Poland lost all of these territories after World War II, during which the Soviets reclaimed all of Ukraine and Belarus in exchange for Polish gains in Germany.